The direct oxidation of ethylene to ethylene oxide is unique in that only silver has been found to be an effective heterogeneous catalyst. Under favorable reaction conditions, when the ethylene conversion is low and mass transfer limiting processes are absent, an efficient catalyst can produce a selectivity in excess of 80%. The selectivity is defined to be the ratio of moles of ethylene converted to EO divided by the total ethylene converted; the primary by-product is carbon dioxide.
Commercially, EO is usually formed under conditions of elevated temperature (200.degree.-300.degree. C.) and pressure (150-300 psig). Two major commercial processes exist. The air process is characterized by a low feed concentration of ethylene (&lt;10%) but operates with high ethylene conversions (20-40%); the oxygen process utilizes high ethylene feed concentrations (20-40%) but operates at low ethylene conversions (5-20%). Since, in general, the selectivity decreases with increasing ethylene conversion, the oxygen process shows higher efficiency. The commercial selectivities appear to be in the range of 65-80% depending on operating conditions.
Although the heterogeneous oxidation of E to EO occurs uniquely in the presence of silver, the physical form of the catalyst and presence or absence of impurities have significant effects upon both the activity of the catalyst and the selectivity obtained from the reaction. Silver in the bulk form has been used as an evaporated film, powder, foil, and alloy in various studies. The use of silver in any of these forms has the advantage of promoting high heat transfer rates, enabling rapid removal of the exothermic heat of reaction and thereby helping to prevent over-oxidation. Use of bulk silver, however, is not commercially acceptable because of low specific activity requiring large reactor volumes and large amounts of the expensive metal. Consequently, porous ceramic bodies, known as catalyst carriers, have been used to permit high dispersion of the silver and thereby enable a much more efficient utilization of silver.
The emphasis in carrier selection has been to provide bodies with suitable porosity (40-60%) compatible with mechanical strength and with a low surface area (0.1-1 M.sup.2 /gm) to insure the absence of strong diffusional resistances for reactants and product gases under reaction conditions. Apart from these physical considerations, the carrier has been required to be inert as far as its catalytic role is concerned. The vast majority of the catalyst patent literature emphasizes the need for an inert carrier. The materials most often used or specified in the catalyst art are alpha-alumina, silica, silicon carbide, and zirconia; although the vast majority of carriers appear to be made from alumina or alumina with small amounts of silica.
The definition of carrier inertness is open to considerable discussion since the presence of low levels of impurities in the carrier may have a beneficial promoter effect on the catalyst performance. In fact, many of the promoters claimed in the catalyst art which are co-deposited with silver on the carrier are already naturally present in the carrier. Typical examples of such promoters are the alkali metals and alkaline earths. Although the role of the promoters is not well defined, it appears that they may act to prevent agglomeration of the finely dispersed silver, cover up surface defects, and inhibit other reaction paths. It is true that in some instances, the majority of these naturally occurring promoters may not be on the accessible carrier surface but may be chemically bound in the bulk of the carrier structure.
A vast catalyst art exists for the deposition of silver on the catalyst carrier. The techniques usually involve either spraying, impregnation, evaporation or precipitation followed by a calcination step in either a reducing or oxidizing environment to activate the silver catalyst. Of these techniques, it appears that impregnation of the carrier with a solution of a suitable silver compound and various promoters is most commonly employed. Typical impregnating solutions may be prepared from aqueous silver nitrate, or more commonly, silver organic complexes of carboxylic acids and organic amines. Such a typical preparation is described by Neilson in U.S. Pat. No. 3,962,136.
As we have just indicated, the prior catalyst art has stressed the need for a non-participatory catalytic role for the carrier and has focused attention on enhancement of catalyst performance by modifying the amount and type of ingredients deposited on the "inert" surface. We believe that the carrier and deposited silver should be considered as a unique catalytic entity with the carrier surface having a strong catalytic influence on the reaction. As a result of this viewpoint, we have discovered that the addition of certain materials to a typical commercial carrier can greatly improve the activity, selectivity or both of the finished catalyst.